It is often desirable to vary the output of a centrifugal pump. Where a centrifugal pump is utilized to recirculate water to the spray nozzles of an air washer humidifier, one should be able to vary the output of the water pump from substantially zero to the maximum output of which the pump is capable. In this respect, the term "output" applies both to pressure and volume. Since the pressure on a bank of nozzles varies approximately as the square of the volume of water forced through them, it will be seen that if the pressure can be controlled, the volume will also be controlled, and vice versa.
In humidifier applications, the evaporation of water increases the mineral concentration of the water being recirculated. This makes it difficult to use ordinary mechanical throttling valves as their working parts become jammed by scale formations. One approach to this problem is disclosed in U.S. Pat. No. 4,243,070 which pertains to a bypass valve in which the only moving part exposed to the water is an elastomer diaphragm. Unfortunately, this valve does not reduce spray nozzle pressure to substantially zero, and is, therefore, unsatisfactory for many humidifying applications. Also, bypassing a large volume of water back to the pump intake increases the power used by the pump when substantially no water is required to be pumped by the pump.
Another approach to this problem is to use a throttling valve which restricts the water flow at the pump outlet. The disadvantage of this approach is that such valves usually impose a restriction, even when fully open, so that the full output of the pump cannot be utilized. If a diaphragm or elastomer tube valve is used, destructive flutter is experienced because of the large pressure drop across the valve. Most such valves require large mechanical forces to close against the shut-off pressure of the pump, making automatic control difficult.
Another solution is to inject compressed air into the intake of a centrifugal pump. With this method, one can reduce the output of the pump to at least substantially zero. By eliminating all such injection, the maximum output of the pump is attained. This method is described in U.S. Pat. No. 3,958,723. In attempting to use only air injection to control the output of a horizontal-axis centrifugal pump, difficulties will be encountered. The output of the pump will be unstable, and can fluctuate wildly. Injecting a predetermined volume of air into the pump intake will not produce a predictable output as the depth of the water in the sump and other factors will affect the pressure. The air injection method does have the advantage of reducing the power consumed by the pump when air is injected to reduce the output. Further examples of the injection of air into the inlet of a centrifugal pump are also disclosed in U.S. Pat. Nos. 2,798,657, 3,663,117, 4,003,674 and 4,142,825. However, these various air injection devices are not readily controllably variable to the extent usually required to obtain full control of the output of a centrifugal pump, from substantially zero output to substantially full capability output.